The present invention relates generally to radio acoustic sounding systems, and more particularly, is directed to a radio acoustic sounding system (RASS) for use with a FM-CW atmospheric Doppler radar system (FADRS) in order to determine atmospheric temperature profiles.
The radio acoustic sounding system is a known technique for remotely determining atmospheric virtual temperature profiles by combining acoustic and radar techniques. With this system, an acoustic wavefront is tracked by using pulsed Doppler radar. Specifically, the RASS technique uses coherent radar to measure backscattered radiation from fluctuations in atmospheric density to determine the local speed of sound. The fluctuations are induced by an external acoustic source, and the determined local speed of sound of the acoustic wavefront is related to the virtual temperature. The radio acoustic sounding system has been used primarily for measuring boundary layer temperature profiles.
Traditionally, RASS has made use of enhancements in the backscatter by matching the acoustic wavelength of the generated acoustic wavefront to one-half of the radar wavelength, otherwise known as Bragg matching, and by making use of the focusing effect of the spherical wavefronts.
Various acoustic sources have been used, for example, as described in the article "Temperature sounding by RASS with wind profiler radars: a preliminary study", by P. T. May et al. See also "Rass Temperature Sounding Techniques" by R. G. Strauch et al., United States Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), Environmental Research Laboratories, Boulder, Colo., NOAA Technical Memorandum ERL WPL-158, Dec. 1988 (deals only with pulsed Doppler radar systems); the doctoral dissertation "The Theory and Application of the Frequency Modulated, Continuous Wave Doppler Radar" by Richard G. Strauch, University of Colorado, 1976; and the book FM Radar by David Luck, McGraw-Hill Publishing, New York, N.Y., 1949.
When a CW acoustic source is used, the backscattered electromagnetic signal will have a frequency shift equal to the transmitted acoustic frequency regardless of the Bragg frequency. Thus, temperature cannot be measured with CW acoustic excitation.
Another acoustic source that has been used produces short acoustic pulses. A short acoustic pulse is one that lies within the radar range volume during the entire observing period, which is typically longer than 0.5 seconds. Because the frequency of the backscattered signal will be Doppler shifted, the backscattered frequency will be the Bragg frequency, regardless of the frequency of the acoustic pulse. As a result, temperature can be determined. However, it has been shown that the influence of horizontal winds in such case will shift the backscattered frequency towards the acoustic frequency for wind speeds in excess of above 10 m/sec.
Still another source of acoustic wave uses long acoustic pulses, in which only a part of the acoustic pulse lies within the resolution volume. The mean frequency of the backscattered signal will lie somewhere between the values for CW acoustic waves and short pulse acoustic waves, for example, 300 m long acoustic pulses. However, because there is a boundary of the acoustic pulse travelling within the radar volume and a part of the acoustic signal intersecting an edge of the radar volume at all times, the spectral signal has two peaks, one at the acoustic frequency and one at the Bragg frequency. Further, if the pulses are repetitive within the observation time or if there is more than one pulse within the radar range volume, additional spectral lines are produced that limit the accuracy to which the Doppler shift (temperature) can be determined.
Finally, a frequency-modulated continuous wave (FM-CW) has been used as the acoustic wave. This acoustic signal has a constant amplitude in which the frequency is modulated by linearly sweeping the acoustic frequency in a sawtooth pattern between 860 and 900 Hz. With this signal, a sharp spectral peak at the Bragg frequency is produced. Thus, only the acoustic energy with a frequency close to the Bragg condition, produces a strong signal. It has been found that a relatively broad frequency sweep can be employed to match the Bragg condition, at all desirable heights simultaneously, while suffering only a minor penalty in the backscatter power.
There are, however, a number of practical considerations with an FM-CW acoustic source. For example, the sawtooth frequency sweep does not put equal power in all frequencies. Thus, the sweep parameters must be varied to produce a smooth averaged spectrum. The magnitude of the variations is proportional to the time-bandwidth product of the frequency sweep. Also, like the pulsed acoustic case, the sweep must not be repetitive within the radar range volume or during the observing period, since otherwise, contaminating spectral lines are produced, thereby degrading the temperature estimates.
The FM-CW acoustic case is the most desirable since the mean and peak powers are the same, and thereby, more efficient use is made of the acoustic transmitter.
However, in the above known systems, a pulsed Doppler radar source has been utilized.
With conventional RASS systems that use pulsed Doppler radar, the minimum altitude in which the temperature can be measured is approximately 100 m. In many situations, however, it is necessary to measure the temperature profile in much lower altitudes, such as for use with air pollution, transport and modeling, for example, in measuring plumes from a smoke stack. Therefore, conventional pulsed Doppler radar RASS is not satisfactory for this purpose.
Further, and related thereto, with conventional RASS systems using pulsed Doppler radar, the spatial resolution is not satisfactory for many commercial applications.
Still further, with such conventional RASS systems, there is the requirement of using a heterodyne receiver in order to perform frequency translations. As a result, the complexity of the system is considerable, requiring additional circuit elements.
Other radio acoustic sounding systems (RASS) have used continuous wave (CW) radar. See, for example, G. Bonino et al., "A metric wave radio-acoustic tropospheric sounder", IEEE Trans. Geosci. Electron., GE-17, pages 179-181, 1979.
Still other systems have used FM-CW radar in conjunction with RASS. See, for example, G. Peters, "Temperature and wind profiles from radar wind profilers equipped with acoustic sources", Meteorol. Rdsch., Vol. 42, pages 152-154, Jun., 1990; G. Peters et al., "Radio Acoustic Sounding of the Atmosphere using a FM-CW Radar", Radio Sci., Vol. 23, No. 4, 1988, pages 640-646; and G. Peters et al., "A Combined RAS-/Radar-System", pages 247 and 248 (date and publication unknown).
In these latter articles, however, no mention was found of deliberately aliasing the output audio signal under a controlled set of radar and digital sampling parameters, so that the homodyne receiver of the FADRS circuitry can be utilized without additional circuitry. Without such aliasing, and assuming use of a homodyne receiver, it would be necessary to use a very high sample rate, thereby making the performance requirements very stringent, and substantially increasing the cost and complexity of the system. If a homodyne receiver is not used, the cost and complexity of the system would increase substantially in the same manner as described above with respect to pulsed Doppler radar RASS.